This invention relates to a data decoder for decoding data transmitted in one or more lines of a field blanking interval of a television signal.
Such decoders are used to decode data at various data rates and having various encoding formats. Examples are teletext and video programming signals (VPS).
Video Cassette Recorder (VCR) control is a new development in the use of broadcast teletext data services. It is now possible to perform two VCR control functions using information contained in broadcast transmissions. The first is the selection of one or more Programmes to be recorded. To make the process of entering the time and channel information into the VCR memory easier and error free, it is possible to programme the VCR from a teletext programme schedule page, where the relevant identification information is held. The second control function is real time switching of the VCR under the control of transmitted data. This enables the desired programme to be recorded despite schedule changes, the VCR being set to record only while the correct broadcast programme label is being transmitted by the broadcaster either as teletext data or as VPS data. Thus, the desired programme may be recorded even when broadcast early or late.
Such transmissions (known as PDC--Programme Delivery Control) are already being provided in the UK and Germany. A VCR equipped with the necessary PDC decoding circuitry is easy to use and gives reliable recording of the chosen items. The technical specification of the PDC system is described in: Specification of the domestic video Programme Delivery Control system (PDC), European Broadcasting Union, August 1990.
There are two different standards for broadcasting the data relevant to each of the two functions. Data required for the above functions is sent on spare TV lines, during the Vertical Blanking Interval (lines 2-22), as either teletext packets or on a dedicated VPS line.
Programme selection information is either incorporated in the displayed programme schedule page as teletext control characters or sent via the teletext extension packets 26. Both methods are based completely on the World System Teletext standard, and the data can be received and decoded using a normal teletext decoder.
Real time switching data, however, may either be sent via the teletext extension packet 8/30/2 or as a special Video Programming Signal (VPS) sent on a dedicated TV line. Teletext and VPS have different data rates (Teletext 6.9375 Mbit/s, VPS 2.5 Mbit/s) and coding schemes as set out in Table 1.
TABLE 1 __________________________________________________________________________ Difference between teletext and VPS transmissions. Characteristic Teletext VPS __________________________________________________________________________ Data Amplitude 0.46 V +/- 10% 0.5 V +/- 5% Position of Start 10.2 .mu.s + 1 .mu.s - 0.4 .mu.s 12.5 .mu.s +/- 1.5 .mu.s of Data Data Rate 6.9375 Mbits/s 2.5 Mbit/s Data Coding Odd Parity, 8/4 Hamming . . . Biphase Data Clock 16 bit run-in 1010 . . . 16 bit run-in Synchronization 1010 . . . Byte Sync Word 27 hex 9951 hex TV Lines Containing 2-22, both fields (VBI) 16, first field Data __________________________________________________________________________
As a result dual standard video recorders or television sets which are required to be able to detect both teletext and VPS data need two data receiver ICs, to cover the whole of Europe.
It is an object of the invention to provide a data decoder which will allow the decoding of data transmitted with a plurality of data rates and/or encoding formats.